1. Technical Field
The present invention relates to a semiconductor device.
2. Description of the Related Art
The degree of integration of semiconductor integrated circuits, in particular, integrated circuits using MOS transistors, has been increasing. With the increasing degree of integration, the size of MOS transistors used in integrated circuits has been decreased to nano-scale dimensions. Such a decrease in the size of MOS transistors causes difficulty in suppressing leak currents, which poses a problem in that it is hard to reduce the area occupied by the circuits because of the requirements of the secure retention of necessary currents. To address the problem, a surrounding gate transistor (SGT) having a structure in which a source, a gate, and a drain are arranged vertically with respect to a substrate and the gate surrounds a pillar-shaped semiconductor layer has been proposed (e.g., Japanese Unexamined Patent Application Publication Nos. 2-71556, 2-188966, and 3-145761).
By using a metal for gate electrodes instead of a polysilicon, the depletion can be suppressed and the resistance of the gate electrodes can be decreased. However, the production process after a metal gate is formed needs to be conducted always in consideration of metal contamination due to the metal gate.
In existing MOS transistors, in order to perform both a metal gate process and a high-temperature process, a metal gate last process in which a metal gate is formed after a high-temperature process has been employed in production of actual products (IEDM 2007 K. Mistry et al., pp. 247 to 250). The metal gate last process includes forming a gate using a polysilicon, then depositing an interlayer insulating film, exposing the polysilicon gate by chemical mechanical polishing, etching the polysilicon gate, and depositing a metal. Therefore, a metal gate last process in which a metal gate is formed after a high-temperature process also needs to be employed in SGTs in order to perform both a metal gate process and a high-temperature process. In SGTs, since a pillar-shaped silicon layer is located at a higher position than a gate, a scheme for employing the metal gate last process is required.
To decrease the parasitic capacitance between a gate line and a substrate, a first insulating film is used in existing MOS transistors. For example, in the FINFET (IEDM 2010 CC. Wu et al., 27.1.1 to 27.1.4), the parasitic capacitance between a gate line and a substrate is decreased by forming a first insulating film around a single fin-shaped semiconductor layer and etching back the first insulating film to expose the fin-shaped semiconductor layer. Therefore, such a first insulating film also needs to be used in SGTs to decrease the parasitic capacitance between a gate line and a substrate. In SGTs, since a pillar-shaped semiconductor layer is formed in addition to the fin-shaped semiconductor layer, a scheme for forming a pillar-shaped semiconductor layer is required.
Furthermore, FINFETs in which two transistors are formed from a single dummy pattern has been known (e.g., Japanese Unexamined Patent Application Publication No. 2011-71235). A sidewall is formed around a dummy pattern and a substrate is etched using the sidewall as a mask to form a fin, and thus two transistors are formed from a single dummy pattern.